1. Field of the Invention
The present invention relates to image display devices for displaying color image, such as monitors and projectors. More particularly, the invention relates to an image display device used under the influence of external light, and to an image display device having a large value of luminance in displaying black because of its characteristic.
2. Description of the Background Art
FIG. 29 is a block diagram illustrating a configuration of a conventional image display device. Referring to FIG. 29, the operation of the conventional image display device will be described below. This image display device is made up of an input image processing means 1 and an image display means 3.
Image data Ri, Gi and Bi composed of three-color (RGB) data to be inputted to the image display device are inputted to the input image processing means 1. The inputted image data Ri, Gi and Bi are subjected to an input image processing to be described hereinafter in the input image processing means 1, and then outputted as image data R1, G1 and B1 composed of three color data. The image data R1, G1 and B1 from the input image processing means 1 are sent to the image display means 3. In the image display means 3, according to the corresponding image data value, each pixel emits a light for image display. Examples of the image display means are a liquid crystal panel and CRT.
FIG. 30 is a block diagram illustrating a configuration of the input image processing means 1 in FIG. 29. Referring to FIG. 30, the input image processing means 1 is made up of a pixel number transforming means 101, a color transforming means 102 and a gradation transforming means 103.
The operation of the input image processing means 1 will be described below. Image data Ri, Gi and Bi inputted to the input image processing means 1 are then inputted to the pixel number transforming means 101 in which, in order to match the number of display pixels in the image display means 3, a pixel number transformation processing of these image data is performed and outputted.
The output from the pixel number transforming means 101 is then inputted to the color transforming means 102 in which a color transformation processing is performed in consideration of the color reproduction characteristic of the image display means 3. This color transformation processing enables to realize a display at a desirable color reproduction in the image display means 3.
The output from the color transforming means 102 is then inputted to the gradation transforming means 103 in which according to the characteristic of the image display means 3, a gradation correction processing is performed and outputted as image data R1, G1 and B1. The pixel number transforming means 101, color transforming means 102 and gradation transforming means 103 may be configured with hardware or software.
Description will now be made of the relationship between the size of image data R1, G1 and B1 inputted to the image display means 3 and the color (light) displayed on the image display means 3. Tristimulus values based on the CIE XYZ colorimetric system of the color (light) displayed on the image display means 3 when image data R1, G1 and B1 are inputted to the image display means 3 under no influence of external light (hereinafter referred to simply as “tristimulus values”) are taken as X1, Y1 and Z1. Assuming the image display means 3 be an image display means in which the relationship between the size of image data R1, G1 and B1 to be inputted and the tristimulus values X1, Y1 and Z1 of the color (light) to be displayed is expressed by the following equation (1).
                              [                                          ⁢                                                    X1                                                                    Y1                                                                    Z1                                              ⁢                                          ]                =                                            [                                                          ⁢                                                                    axr                                                        axg                                                        axb                                                                                        ayr                                                        ayg                                                        ayb                                                                                        azr                                                        azg                                                        azb                                                              ⁢                                                          ]                        [                                                  ⁢                                                            R1                                                                              G1                                                                              B1                                                      ⁢                                                  ]                    +                      [                                                  ⁢                                                            Xbk1                                                                              Ybk1                                                                              Zbk1                                                      ⁢                                                  ]                                              (        1        )            wherein Y1 is a value corresponding to a luminance.
In equation (1), axr, ayr, azr, axg, ayg, azg, axb, ayb and azb, and Xbk1, Ybk1 and Zbk1 are values that depend on the characteristic of the image display means 3. Particularly, Xbk1, Ybk1 and Zbk1 are tristimulus values of the color (light) displayed on the image display means 3 when the image display means 3 displays black under no influence of external light, that is, when R1=G1=B1=0. Hereat, let axr, ayr, azr, axg, ayg, azg, axb, ayb and azb be a chromaticity data indicating the correlation between color data RGB in the image display means 3 and be tristimulus values XYZ, and a value expressed by the following equation (2).axr=0.4124, axg=0.3576, axb=0.1805,ayr=0.2126, ayg=0.7152, ayb=0.0722,azr=0.0193, azg=0.1192, azb=0.9505  (2)
Let image data R1, G1 and B1 inputted to the image display means 3 be an integer and a value in the range expressed by the following equation (3).0≦R1≦1000≦G1≦1000≦B1≦100  (3)
Theoretically, all the tristimulus values Xbk1, Ybk1 and Zbk1 in displaying black should be “0”, however, they have in fact a value larger than “0”. The tristimulus values of a reflected light caused by that the surface of the image display means 3 is irradiated with an external light and the external light is reflected from the surface of the image display means 3, are taken as X2, Y2 and Z2. In this case, the tristimulus values X3, Y3 and Z3 of the light entering the eyes of a viewer of the image display device are expressed by the sum of the tristimulus values X1, Y1 and Z1 of the color displayed on the image display means 3 by input signals R1, G1 and B1, and the tristimulus values X2, Y2 and Z2 of the reflected light. That is, X3, Y3 and Z3 are expressed by the following equation (4). To the viewer, the color expressed by X3, Y3 and Z3 seems as if the color expressed by X3, Y3 and Z3 was displayed on the image display means 3.
                              [                                                    X3                                                                    Y3                                                                    Z3                                              ]                =                                            [                                                                    X1                                                                                        Y1                                                                                        Z1                                                              ]                        +                          [                                                                    X2                                                                                        Y2                                                                                        Z2                                                              ]                                =                                                    [                                                                            axr                                                              axg                                                              axb                                                                                                  ayr                                                              ayg                                                              ayb                                                                                                  azr                                                              azg                                                              azb                                                                      ]                            ⁡                              [                                                                            R1                                                                                                  G1                                                                                                  B1                                                                      ]                                      +                          [                                                                                          Xbk1                      +                      X2                                                                                                                                  Ybkl                      +                      Y2                                                                                                                                  Zbkl                      +                      Z2                                                                                  ]                                                          (        4        )            
From equation (4), Xbk1+X2, Ybk1+Y2, and Zbk1+Z2 are tristimulus values in displaying black on the image display means 3, taking the influence of external light into consideration. From equation (4), the variations in tristimulus values Xbk1, Ybk1 and Zbk1 in displaying black under no influence of external light, and the variations in tristimulus values X2, Y2 and Z2 of a reflected light of external light, exert the same influence on the tristimulus values X3, Y3 and Z3 of the light entering the eyes of the viewer. Therefore, description will now be made of the case that the values of Xbk1, Ybk1 and Zbk1 are fixed and the values of X2, Y2 and Z2 vary due to the influence of external light. The same is true for the case that the values of Xbk1, Ybk1 and Zbk1 vary. Hereat, Xbk1, Ybk1 and Zbk1 are values expressed by the following equation (5).Xbk1=1Ybk1=1Zbk1=1  (5)
FIG. 31 is an explanatory diagram showing in the form of table the relationship between R1, G1 an B1 inputted to the image display means 3 and tristimulus values X3, Y3 and Z3 of the color (light) entering the eyes of a viewer, under no influence of external light, i.e., when X2=Y2=Z2=0. FIG. 31 indicates the case that the relationship of R1=G1=B1 holds, i.e., that an achromatic data is inputted to the image display means 3.
Referring to FIG. 31, consider the case of having no influence of external light. No influence of external light results in X2=Y2=Z2=0. When the maximum values 100, 100 and 100 of image data R1, G1 and B1 are inputted to the image display means 3, the tristimulus values of the color (light) entering the eyes of a viewer in the absence of external light are X1=96.05, Y1=101 and Z1=109.9. On the other hand, when the minimum values 0, 0 and 0 of image data R1, G1 and B1 are inputted to the image display means 3, tristimulus values of the color (light) entering the eyes of the viewer in the absence of external light are X1=1, Y1=1 and Z1=1.
In FIG. 31, in the tristimulus values of the color (light) entering the eyes of a viewer when R1, G1 and B1 are inputted to the image display means 3, the ratio of a Y3 corresponding to a luminance to a Y3 when R1=100, G1=100 and B1=100 (i.e., in displaying white) is indicated as a ratio to white (Y/Ymax). As the ratio to white is smaller with respect to each image data, the viewer seems that the image displayed on the image display means 3 has a larger contrast and more excellent visibility.
FIG. 32 is a graph showing the relationship between image data R1, G1 and B1 inputted to the image display means 3 and a luminance stimulus value Y3.
Next, description will be made of an image display on an image display means 3 of a conventional image display device when the device is used under the influence of external light.
FIG. 33 is an explanatory diagram showing in the form of table the relationship between R1, G1, B1 and tristimulus values X3, Y3, Z3 of the color (light) entering the eyes of a viewer under the influence of external light. FIG. 33 indicates that the relationship of R1=G1=B1 holds, i.e., that an achromatic data is inputted to the image display means 3.
Let tristimulus values of a reflected light of external light on the surface of the image display means 3 be X2=0.505, Y2=10, and Z2=10.89. When the maximum values 100, 100 and 100 of image data R1, G1 and B1 are inputted to the image display means 3, tristimulus values of the color (light) entering the eyes of a viewer are X3=105.555, Y3=111.000 and Z3=120.790. On the other hand, when the minimum values 0, 0 and 0 of R1, G1 and B1 are inputted to the image display means 3, the tristimulus values of the color (light) entering the eyes of the viewer are X3=10.505, Y3=1.000 and Z3=11.890.
Also in FIG. 33, in the tristimulus values of the color (light) entering the eyes of a viewer when R1, G1 and B1 are inputted to the image display means 3, the ratio of a Y3 corresponding to a luminance to a Y3 when R1=100, G1=100 and B1=100 (i.e., in displaying white) is indicated as a ratio to white (Y/Ymax). As compared to the case having no influence of external light shown in FIG. 31, under the influence of external light, the resulting values are large as a whole. That is, under the influence of external light, the viewer regards the image as having a small contrast and poor visibility.
FIG. 34 is a graph showing the relationship between image data R1, G1 and B1 inputted to the image display means 3 and a luminance stimulus value Y3. In FIG. 34, a continuous line represents the case having the influence of external light, and a dotted line represents the case having no influence of external light.
In order to overcome a decrease in contrast due to the influence of external light, it can be considered that the brightness of display on the image display means 3 is increased under the influence of external light. For instance, doubling the brightness of display on the image display means 3 doubles the values of tristimulus values X1, Y1 and Z1 of the color (light) displayed on the image display means 3.
FIG. 35 is an explanatory diagram showing in the form of table the relationship between R1, G1 and B1 and tristimulus values X3, Y3 and Z3 of the color (light) entering the eyes of a viewer under the influence of external light while doubling the brightness of display on the image display means 3. FIG. 35 indicates that the relationship of R1=G1=B1 holds, i.e., that an achromatic data is inputted to the image display means 3. As in the case of FIG. 33, the tristimulus values of a reflected light of external light on the surface of the image display means 3 are taken as X2=9.505, Y2=10, and Z2=10.89.
Also in FIG. 35, in the tristimulus values of the color (light) entering the eyes of a viewer when R1, G1 and B1 are inputted to the image display means 3, the ratio of a Y3 corresponding to a luminance to a Y3 when R1=100, G1=100 and B1=100 (i.e., in displaying white) is indicated as a ratio to white (Y/Ymax). As compared to FIG. 35, by doubling the brightness of display on the image display means 3, the ratio to white can be closer to that in FIG. 31 having no influence of external light, however, it is still large as compared to FIG. 31.
In order to increase the brightness of display on the image display means 3, there are the following methods of increasing the brightness of display on the image display means 3 itself without changing the data inputted to the image display means 3, or adjusting the gain of image data R1, G1 and B1 inputted to the image display means 3 without changing the brightness of display on the image display means 3 itself.
In the former method, it is very difficult to double the brightness of display on the image display means 3 as described above, in terms of cost, power consumption and useful life.
In the latter method, it is impossible to increase the gain limitlessly because of the upper limit of image data R1, G1 and B1, and a larger gain causes a larger disarrangement of gradation.
Thus, in the conventional image display device, the influence of external light or the event that the luminance in displaying black is large due to the characteristic of the image display means, may cause a considerable large ratio to white (Y/Ymax), which is a ratio of a luminance displayed for each image data to a luminance in displaying white. Accordingly, the viewer feels that the image has a small contrast and poor visibility.
In addition, in terms of cost, power consumption and useful life, it is very difficult to suppress an increase of the ratio to a luminance in displaying white, by increasing the brightness of display on the image display means. This results in poor improvement.